**2.2. Fish collection and homogenization**

Two species of fish (mullets, *Mugil curema*; *n* = 11; and, Galapagos thread herrings, *Ophistonema berlangai*; *n* = 4), which for the purpose of this study were assumed to be major prey items of Galapagos sea lions, were collected from Galapagos waters by fishers during March-April 2008. Mullets are coastal fish, inhabiting nearshore habitats, and demersalbenthic feeders (detritivorous), grazing on detritus and bottom sediments and digesting the nutritive matter (iliophagous foraging), while Galapagos thread herrings are endemic, pelagic and schooling fishes that filter-feed (planktivorous) mainly on tiny planktonic organisms (e.g., phytoplankton) in open waters [36].

After field collection, fish specimens were frozen until further transportation to the lab, where they were stored at -80ºC. Each fish was measured, weighed and sexed. Muscle biopsies were extracted from the dorsal, lateral muscle of each fish, using a 6mm–biopsy punch (Accuderm, USA), and saved in vials for stable isotope analysis.

Each individual fish was homogenized using a clean, hexane-acetone rinsed meat grinder (Omcam Inc., Italy). The ground fish was then further homogenized in a homogenizer (Omni, USA and/or Polytron, Kinematica, GmbH, Switzerland) at dial position 5-6 for ≈1 min until material was well mixed and homogenous in appearance. Homogenized samples and subsamples were transferred to clean glass jars and stored at -80 ºC until further chemical analysis.
